This invention relates generally to lightwave communication networks and, more particularly, to optical demultiplexing units used in wide band dense wavelength division multiplexed (DWDM) systems.
Wavelength Division Multiplexing (WDM) increases the capacity of lightwave communication systems by multiplexing many optical channels of different wavelengths for transmission as a composite signal in an optical fiber. At present, most WDM systems deployed in communication networks are generally considered to be low capacity systems, e.g., 4, 8, and 16-channel systems. With recent advances in optical networking technology, system manufacturers are now contemplating dense wavelength division multiplexing (DWDM) systems having as many as 80 channels, for example. One technique for increasing the number of channels in a DWDM system is to reduce channel spacing between adjacent channels. For example, 50 Ghz channel spacing may be used to pack more channels in a DWDM system as compared with the more conventional 100 Ghz channel spacing.
Optical demultiplexers are generally used in WDM systems for demultiplexing a multi-wavelength composite signal into individual optical channels of different wavelengths. One example of an optical demultiplexer is a waveguide grating router described in U.S. Pat. No. 5,002,350 issued to Dragone on Mar. 26, 1991. In general, waveguide grating routers and other types of optical demultiplexers used in existing WDM systems are not well-equipped for meeting the technological demands of DWDM systems that have a large number of closely-spaced optical channels.
Some of the problems associated with demultiplexing channels in a DWDM system include increased crosstalk, larger device size requirements, larger free spectral range (FSR), and random power divergence in the optical channels. More specifically, total system crosstalk increases as a function of the number of channels in the DWDM system. The increase in crosstalk is mainly attributable to the increased crosstalk contribution from non-adjacent channels. For example, crosstalk experienced in one of the channels of an 8-channel system is a function of crosstalk from two adjacent channels as well as crosstalk contribution from 5 non-adjacent channels. By contrast, crosstalk experienced in one of the channels of an 80-channel system is a function of crosstalk from two adjacent channels as well as crosstalk contribution from 77 non-adjacent channels. Consequently, the increased number of non-adjacent channels will dominate the total crosstalk in DWDM systems.
Crosstalk in optical demultiplexers is also sensitive to power divergence among the optical channels. In general, power divergence in lower capacity systems, e.g., 8-channels, can be compensated more easily than the random power divergence found in higher capacity DWDM systems, e.g., 80-channels, where gain flattening schemes are used. Another problem to overcome for optical demultiplexers in DWDM systems is the larger device size needed to accommodate the additional channels. At present, device size is limited by current technologies, such as filter technologies and the like. To support the large number of channels in DWDM systems, optical demultiplexers also require a larger free spectral range (FSR) than that provided in existing devices.
Substantial reduction in crosstalk and improved scalability for supporting high channel counts in dense wavelength division multiplexed (DWDM) systems is achieved according to the principles of the invention in an optical demultiplexer arrangement that partitions the total number of input optical channels into separate demultiplexer modules, demultiplexes smaller groups of optical channels in the separate demultiplexer modules, and filters the individual optical channels at the outputs of the separate demultiplexer modules. Partitioning the total number of channels into smaller demultiplexing groups and post-filtering a reduced number of demultipexed optical channels reduces the number of non-adjacent channels that can contribute to the total crosstalk level. Accordingly, crosstalk is substantially reduced as compared with prior arrangements. Furthermore, the modularity of the optical demultiplexer arrangement results in smaller device footprints and smaller free spectral ranges associated with the demultiplexer modules. This modularity also allows for future system upgrades without redesign and without disruption to existing service.
In one illustrative embodiment, the optical demultiplexer arrangement includes a splitter or filter for directing the multi-wavelength input optical signal to at least two demultiplexer modules. Each of the demultiplexer modules can be a waveguide grating router or other suitable optical demultiplexer which separates a received multi-wavelength optical signal into individual wavelength channels. The individual wavelength channels are then supplied to bandpass post-filters which are coupled to each of the outputs of the demultiplexer modules. In one embodiment, a separate filter corresponds to each of the outputs. In another embodiment, a filter may be coupled to a plurality of outputs to receive and filter more than one wavelength channel.